If you have recently been curious as to what that number 23 next to a smiling, cutesy electrical outlet meant, the secret is now out. The cryptic ad campaign was the new GM’s way of dropping hints to the expected 230-mpg city rating for the 2011 Chevrolet Volt.

In a press conference this morning, CEO Fritz Henderson discussed the new methodology under development by the EPA for rating plug-in hybrids that leads GM to expect a 230-mpg city score for the range-extended electric Volt. This new system weights vehicle ratings based on the expectation that they will travel more city miles than highway miles on electricity only. To define the electrical efficiency of plug-ins, the EPA calculates the kilowatt hours used per 100 miles traveled. GM is expecting the Volt, which is said to travel up to 40 miles using only electricity stored in its 16-kWh lithium-ion battery pack, to use as little as 25 kilowatt hours per 100 miles in the city. Using a national average of about 11 cents per kWh, GM anticipates the average Volt customer would pay about $2.75 for electricity to travel up to 100 miles.

Henderson is withholding a highway fuel-economy rating but said it and the overall combined score will also be triple-digit numbers. Pricing for the Volt is still up in the air but many expect to see it top the $40,000 mark, although a government tax credit of $7500 is projected. The 230-mpg city fuel economy estimate further adds to the mountain of hype surrounding the Volt and Henderson assures that GM is not “over-promising” on the numbers. We sure hope GM delivers, in these times we all know it can’t afford to fall short of its promises.